Emissions from fossil fuel-fired power plants represent a significant source of carbon dioxide emissions, a known greenhouse gas. To significantly reduce CO2 emissions from such power plants, CO2 must be captured, compressed and transported to a sequestration site. One approach for capturing CO2 from a conventional coal-fired boiler uses an amine-based solution to absorb CO2 from the flue gas stream.
FIG. 1 is a schematic of a gas-treating system of a coal-fired plant utilizing a typical alkanolamine treating unit. The feed gas containing CO2 flows into the bottom of the absorber where it contacts an amine solution. The CO2 gas component is removed from the gas stream by chemical reactions with the amine which flows counter-concurrently. The purified gas is the overhead product path, while the rich amine solution is removed from the bottom of the absorber to the rich amine flash drum. In the flash drum, the rich amine flashes at a lower pressure to remove dissolved and entrained hydrocarbons. The rich amine then flows from the flash drum through a lean/rich amine heat exchanger and on to a stripper/amine regenerator. In the stripper/amine regenerator, the CO2 gas component is stripped from the solution using heat supplied by the regenerator reboiler. CO2 gas is the amine regenerator overhead product while lean amine solution is removed from the bottom of the regenerator. The hot lean amine from the regenerator is heat exchanged with the rich amine, cooled and then returned to the absorber.
Removing CO2 with amine solvent results in corrosion problems in apparatus that is in contact with the amine solution. The equipment and piping in an amine plan are usually made of steel, either carbon steel or stainless steel. It is common practice to utilize stainless steel in locations of the separation plant where carbon steel would corrode resulting in a failure. Carbon steel can be subject to uniform and localized corrosion such as pitting, galvanic corrosion, erosion, stress cracking and intergranular corrosion when in contact with amine solutions having carbon dioxide and other impurities from the exhaust gases resulting from the combustion of coal. Carbon steel is generally used for the absorber, lean amine tubes, the reflux drum, and the regenerator shell. Stainless steel, such as austenitic 304 stainless steel, is used for the absorber, regenerator internals, rich-lean heat exchangers, pump impellers, and rich amine piping.
While stainless steel has a particularly long life, it is very expensive and therefore not desirable. Because the amine solution is used for the capture of CO2 from a combustion gas, the amine solution usually contains additional impurities in solution that further shorten the life of carbon steel when the amine solution is contacted to it, further making carbon steel a less than desirable choice for use in a separation plant and increasing the cost of the plant. What is desirable is an amine solution that can be used with carbon steel such that the carbon steel in contact with amine solution has a reduced corrosion rate.